The present invention relates to a hydraulic control system for controlling a solenoid operated valve in a hydraulic circuit of an automatic transmission for a motor vehicle, and more particularly to the system for correcting a dither applied to a control current for the solenoid operated valve.
A control system for a continuously variable belt-drive automatic transmission (CVT) comprises an endless belt running over a primary pulley and a secondary pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary a running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, and a pair of solenoid operated control valves for controlling secondary pressure and primary pressure in the circuit.
The system is employed with an electronic control system for controlling the valves of the hydraulic circuit. The solenoid operated control valve is a type of a proportional solenoid operated valve or reducing valve which controls hydraulic pressure in proportion to control current applied to a solenoid. Such a valve has hysteresis characteristics caused by friction between a spool and a valve body in the valve as shown by dotted lines c of FIG. 3a. In order to reduce the hysteresis of the valve, the dither which cyclically oscillates at a predetermined frequency fd with an amplitude Ad is added to a basic exciting current I to produce a dither exciting current Id as shown in FIG. 3b. The exciting current is applied to the solenoid. Thus, the hysteresis is reduced as shown by solid lines d of FIG. 3a.
However, if the viscosity of the oil changes in accordance with the temperature of the oil, the magnitude of the hysteresis also changes.
Japanese Patent Application Laid-open 1-199079 discloses a control system of a proportional solenoid operated reducing valve, where the frequency of the dither current is changed in a high temperature range and a low temperature range to reduce the change of the hysteresis.
However, in the valve, if the rotating speed of the oil pump is increased, the flow rate of the oil passing through the valve increases. A large pressure of the oil is applied to the spool of the valve in a radial direction of the spool to urge the spool to an inside wall of the valve body. As a result, the variation value of the control pressure corresponding to the displacement of the spool and the control gain become small. Accordingly, the effect of the dither current is not sufficiently obtained. Since the hysteresis increases, a preferable static characteristic between the solenoid current and the secondary pressure can not be obtained, causing malfunctioning and slipping of the belt of the transmission. Furthermore, the excessive pressure is fed to a hydraulic cylinder.